Move from miniswig to Simple Squirrel (SuperTux fork)

[tulpenkiste]

I personally believe miniswig makes working with the brux codebase quite daunting and not at all a fun experience to work with. Not too long ago, the SuperTux project hard forked SimpleSquirrel (along with also preparing to ditch miniswig in favour of simplesquirrel) and, imo, using SimpleSquirrel makes dealing with Squirrel a lot more understandable and easier while not having particularly many trade offs.